(Formerly JP4.4) Some 3D aspects of a Striated Cloud Head over New Zealand including radar and raingauge data

One of the most damaging extratropical cyclones in terms of flooding and severe winds over central New Zealand occurred on 15/16 February 2004, late summer in the Southern Hemisphere. Onset of the widespread and prolonged heavy rain (return period up to 150 years or more) over the southern North Island was associated with a warm conveyor belt and striated cloud head capped by a dry intrusion aloft. The striations took the form of a wavelike modulation of the cloud tops in and carried along by, an ascending warm conveyor in the cloud head. Wavelength and amplitude increased rapidly with time and distance downstream. Model diagnostics suggested that maximum broadscale ascent within the conveyor was about 3km when the striations were largely confined between about 5 and 8km.

The striated cloud head occurred partly within weather radar coverage. Animated 15 minute PPI and cross sections revealed moving bands of echoes with the strongest returns coincident with the more prominent striations in the cloud tops. Rainfall data at 6 minute intervals from several automatic raingauges showed peaks in intensity which coincided with the passage of at least two of these bands. This suggests that the vertical motions causing the striations were not confined to cloud top level but were spread over a substantial depth of the troposphere.

MM5 which is used operationally at MetService of New Zealand was re-examined using Vis5D to see if it could resolve banding in precipitation and/or cloud water. It showed increasing depth of cloud water roughly following the warm conveyor and several transverse bands stacked in a slantwise fashion but these were stationary.